As semiconductor devices are continuously scaled down, the wafer cleaning techniques have been diversified and increasingly significant. Particularly in a process for fabricating semiconductor devices having a fine structure, contaminants and particles attached to a cleaned wafer, static electricity, water marks, and linear particles have a great effect on subsequent processes.
In the fabrication of semiconductor devices, numerous processing steps, as many as several hundred, must be executed on a silicon wafer. These steps typically include (1) deposition of conductive and insulative layers on the silicon wafer substrate, (2) chemical or polishing process, and (3) application of a photoresist (4) mask such as titanium, oxide or silicon oxide, (5) lithographic or photolithographic techniques, (6) etching removing and stripping. Fabrication processes as those listed leave unwanted residual on the wafer surfaces. If left on the wafer, the unwanted residual material and particles may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is necessary to clean the wafer thoroughly yet efficiently after processing steps that leave unwanted residues. Since the processing of silicon wafers requires extreme cleanliness in the processing environment to minimize the presence of contaminating particles or films, the surface of the wafer is frequently cleaned after each processing step. After a wafer has been wet cleaned, it must be dried effectively to prevent water or cleaning fluid remnants for leaving residues on the wafer. If droplets of cleaning fluid form on the wafer surface, residue or contaminants previously dissolved in the cleaning fluid will remain on the wafer surface after evaporation forming spots. To prevent evaporation from taking place, the cleaning fluid must be removed as quickly as possible to prevent the formation of droplets on the wafer surface which subsequently interact with the base silicon, form oxides, and leave microscopic etched areas referred to as “water stains.”
In an attempt to accomplish the cleaning and drying process without leaving contaminants, several drying techniques may be employed. Techniques currently utilized include spin drying, IPA (isopropyl alcohol) drying and Marangoni drying. It has been found the wafer cleaning by immersion or soaking of the wafer in deionized water (DI water); followed by drying of the wafer using IPA is effective in removing both particles and surface electrostatic charges from the wafer. See U.S. Pat. No. 6,926,590 B1. All of the drying techniques utilize some form of a moving liquid/gas interface on a wafer surface, which, if properly maintained, results in drying of a wafer surface without the formation of droplets. Unfortunately, if the moving liquid/gas interface breaks down, as often happens with all of the drying methods, droplets form and evaporation occurs resulting in contaminants being left on the wafer surface. See U.S. Pat. No. 6,616,772 B2.
In addition, these processes have difficulties with drying surfaces that are hydrophobic. Hydrophobic wafer surfaces can be difficult to dry because such surfaces repel water and water based (aqueous) cleaning solutions. Therefore, as the drying process continues and the wafer is pulled away from the cleaning fluid, the remaining cleaning fluid on the wafer (if aqueous based) will be repelled by the hydrophobic surface. As a result, the aqueous cleaning fluid will want the least amount of area to be in contact with the hydrophobic wafer surface. Additionally, the aqueous cleaning solution tends to cling to itself as a result of surface tension (i.e., as a result of molecular hydrogen bonding). Therefore, because of the hydrophobic interactions and the surface tension, droplets of aqueous cleaning fluid form in an uncontrolled manner on the hydrophobic wafer surface. This formation of droplets results in the harmful evaporation and the contamination previously discussed.
A typical wafer cleaning process consists of a DI water spray followed by a spin dry and nitrogen gas blow dry step. More recently, the solvent drying technology utilizes the use of isopropyl alcohol (IPA). The IPA vapor drying process is normally controlled by three major elements, i.e. the purity and water content of IPA; the flow rate and flow speed of the IPA vapor; and the cleanliness of the IPA vapor. The improved solvent drying technique in the past few years is Marangoni drying. In a Marangoni dryer, the drying principal is based on the different surface tension of IPA and DI water. The different surface tension causes the reduction of water molecules at the wafer surface. The Marangoni drying process is carried out by slowly withdrawing the wafer from a DI water tank. At the same time, IPA vapor carried by N2 carrier gas is directed into the water chamber where IPA interacts at the water surface and diffuses into the water such that IPA is saturated on the exposed wafer surface above the water level. Since the concentration of IPA on the surface of the exposed wafer is larger than the concentration of DI water, the surface tension of IPA is reduced as compared to the surface tension of water in the water tank. This causes the water molecules on the surface of the exposed wafer to be retracted into the water tank and thus achieving the drying purpose. See U.S. Pat. No. 6,401,361 B1. The Marangoni drying process also prevents the formation of static electricity on the wafer, and thus, prevents the adherence of small charged particles to the wafer. As a result, the yield of IC devices fabricated on the wafer, as well as the performance of the devices, is enhanced.
As described in some of the examples above, referred to as surface finishing, many recognize these practices to achieve a level of art in semiconductor manufacturing relating to substrate texture and morphology. These practices may range from a deposition or removal process to cleaning. Such processes are conducted inside sophisticated tools/equipment designed for their precision to deliver and rinse ultrapure chemicals. Although such tools are ideal for control, they can be limited in their flexibility for new developments. To aid the screening of candidates for later testing on a tool, acceptable laboratory models are needed. This need is also stated by the ITRS in Modeling and Simulation, where demands are made for more predictive models for algorithms and simulation programs to allow larger problems to be tackled to include new materials and equipment. See International Technology Roadmap for Semiconductors.
Therefore, there is a need for products and methods allowing quick and efficient cleaning and drying of a semiconductor wafer, but at the same time reducing the formation of water or cleaning fluid droplets which may cause contamination to deposit on the wafer surface. New ways of developing and characterizing such products are also needed in order to accelerate their movement throughout an evaluation. The present invention focuses on the novel use of non-traditional semiconductor solvents to remove residual ionic, moisture, and particle contamination through the use of screening tests conducted with laboratory models.